Conventionally, in an intermeshing type twin screw extruder and a twin screw continuous mixing machine, a mixing degree adjusting device for adjusting a cross section of flow route is provided in the middle of an internal flow route so as to adjust flow of an object to be mixed flowing through the machine and to control a mixing degree of the object to be mixed. Specifically, the intermeshing type twin screw extruder has a structure of extracting screws forward at the time of maintenance, and the mixing degree adjusting device is required to have a structure that is in consideration with the extracting of the screws.
The mixing degree adjusting device for adjusting the cross section of flow route includes a slot bar method, a taper ring method and a gate method.
In the slot bar method, two slot bars formed by providing a half arc surface with a curvature that is equal to or slightly larger than a curvature of an inner peripheral surface of a barrel in a round bar are arranged in the middle of a flow route in the machine so as to lie at right angles to axes of two screws and sandwich the screws from the top and the bottom, and by rotating the two slot bars in the different directions from each other, the cross section of flow route is changed so as to adjust the flow rate of the object to be mixed (Patent Documents 1 and 2).
In the taper ring method, a large outer diameter portion, a small outer diameter portion and a screw side taper portion for connecting the outer diameter portions are provided in the middle of the screws, and in the barrel, a large inner diameter portion is provided at a part opposing to the large outer diameter portion, a small inner diameter portion is provided at a part opposing to the small outer diameter portion, and a barrel side taper portion for connecting the inner diameter portions is provided. By moving the barrel side taper portion in the axial direction of the screws, a clearance between the screw side taper portion and the barrel side taper portion is changed so as to adjust the flow rate of the object to be mixed (Patent Documents 3 and 4).
In the gate method, two gate plates having cutouts in an arc shape with a curvature that is substantially equal to a curvature of inner periphery of the barrel or a curvature of outer periphery of a screw axis at edges opposing to each other are arranged in the middle of the flow route in the machine so as to lie at right angles to axes of two screws arranged horizontally and sandwich the screws from the top and the bottom, and by moving the two gate plates upward and downward in the different directions from each other, the cross section of flow route is changed so as to adjust the flow rate of the object to be mixed (Patent Documents 5 and 6).
However, there is the following problem in the mixing degree adjusting devices of the methods mentioned above.
That is, in the mixing degree adjusting device of the slot bar method disclosed in Patent Documents 1 and 2, even at the time of closing action, a flow route is formed between both end parts in the axial direction of the slot bars of the half arc surface and the screw axes, and thus it is not possible to obtain a sufficient drawing effect. It should be noted that the “sufficient drawing effect” means an effect of sufficiently narrowing the cross section of flow route at the time of totally closing with a wide adjusting range of the mixing degree. Meanwhile, when the curvature of the half arc surface of the slot bars is made smaller than the curvature of the inner peripheral surface of the barrel in order to obtain the sufficient drawing effect, it is not easy to extract the slot bars at the time of maintenance, and there is a fear that the mixing degree adjusting device is required to be disassembled for the extracting.
In the mixing degree adjusting device of the taper ring method disclosed in Patent Documents 3 and 4, since the large outer diameter portion or a flight of the screws and the small inner diameter portion of the barrel interfere with each other, there is a problem that the screws cannot be extracted forward without moving the barrel.
In the mixing degree adjusting device of the gate method, when the cutouts of the gate plates have the curvature that is substantially equal to the inner periphery of the barrel, the adjusting range of the mixing degree is narrowed and it is not possible to obtain the sufficient drawing effect. When the cutouts of the gate plates have the curvature that is substantially equal to the curvature of the outer periphery of a land portion formed smaller than a diameter of the screws as disclosed in Patent Document 5 in order to obtain the sufficient drawing effect, at the time of totally opening the gate plates, a hollow generated by housing the edges in an arc shape of the gate plates to the inside of an inner surface of the barrel easily becomes a large dead space (Patent Document 5 and FIG. 3).
When the cutouts of the gate plates have the curvature that is substantially equal to the curvature of the outer periphery of the land portion formed smaller than the diameter of the screws, following the opening of the gate plates, not only a clearance between the gate plates between the two screws but also a clearance between the gate plates on the left and right outer sides of the two screws are increased so that the cross section of flow route is radically changed. Therefore, there is a problem that fine-adjustment of the mixing degree is not easily performed. With regard to the problem of the fine-adjustment of the mixing degree, in the mixing degree adjusting device disclosed in Patent Document 6, left and right end parts of the gate plates extend to the opposing gate plate side, and inclinations in the up and down direction are provided in the extended parts of the gate plates so as to mesh with each other so that the inclined parts are overlapped (meshed) with each other irrespective of an opening degree of the gate plates and the radical change in the cross section of flow route is prevented. However, since the left and right end parts of the gate plates extend in the up and down direction, a moving stroke of the gate plates for extracting the screw axes (action length) becomes large so that in the twin screw extruder provided with the mixing degree adjusting device of Patent Document 2, particularly due to a need for ensuring a space on the lower side, there is a large limitation on installment thereof.
Patent Document 1: Japanese Patent Laid-Open No. 2000-309018
Patent Document 2: Japanese Patent Laid-Open No. 2003-33961
Patent Document 3: Japanese Patent Laid-Open No. Sho58-87013
Patent Document 4: Japanese Patent Laid-Open No. Hei11-216762
Patent Document 5: Japanese Patent Laid-Open No. 2002-28963
Patent Document 6: Japanese Patent Laid-Open No. Hei10-278099